We are studying the function and structure of viral and tumor antigens and proteins involved in cell differentiation, in cell surface recognition, and in the pathophysiology of skin diseases. A variety of monoclonal and polyclonal sera have been prepared that serve to identify and purify these immunologically and biologically significant proteins. The amino acid sequence of these proteins is essential to an understanding of their biologic functions and provides the information necessary to clone the corresponding genes. A critical problem in this research effort is that most of the interesting proteins occur in very limited amounts (picomoles). A number of Wistar Institute investigators supported by NIH grants cannot perform crucial experiments because they lack the amino acid sequence of these scarce proteins. A highly sensitive protein microsequencer is currently available that would solve these problems. Protein analysis with this microsequencer has a variety of potential applications for problems in protein chemistry, biochemistry and molecular genetics.